If you use recipes that often call for buttermilk, you can have it on hand by buying dried buttermilk. It comes in a round cannister where the other dry milk products are.
I have used it in place of fresh buttermilk in lots of recipes and I can't tell any difference.
Put 1 T. of white vinegar in a measuring cup, add milk to make it one cup. Let it sit for at least 5 minutes. Lemon juice will work but vinegar is preferred.
thanks guys can't tell you how many times I've had to thow out buttermilk because I only used a cup and the rest sat in my fridge so long it went bad. I wonder if it makes a difference using whole milk or 2% when adding the vinegar? But I will definitley give it a try the next time I need buttermilk.
I never buy fresh buttermilk. I always use the dried buttermilk. I get Bob's Red Mill brand. It's good quality and cheaper in the long run (although more $ for the weight) because it's more concentrated than the stuff in the can and you use less. I even have a container just for my buttermilk powder right up there with my cocoa powder container and my flour container 
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